In recent years, image forming apparatus such as copying machines and printers are sought to achieve much higher speed, higher image quality and higher stability as they make progress in their use for various purposes and in various environments. For example, printers, which have ever been chiefly used in offices, have come to be used in severe environments, and it has become important for them to promise stable image quality even in such a case.
In copying machines and printers, the apparatus make progress in being made compact and energy saving at an aim to make them usable without preference for places where they are installed and environments where they are used, and a magnetic one-component developing system, which makes use of a magnetic toner, is preferably used as being advantageous in these respects. In the magnetic one-component developing system, the magnetic toner is held by using a toner carrying member (hereinafter “developing sleeve”) provided in its interior with a magnetic-field generation means such as a magnet roll, and is transported to a developing zone to perform development. The magnetic toner is also provided with electric charges chiefly by triboelectric charging by the rubbing friction between the toner and a triboelectric charge providing member such as the developing sleeve.
In a low-temperature and low-humidity environment, where the magnetic toner tends to be electrostatically charged, a phenomenon called charge-up in which the toner greatly increases in charge quantity may come about to damage developing performance of the toner. That is, any toner having been charged up may remain on the developing sleeve, and this may cause a decrease in image density or may make the whole toner thereon charged non-uniformly to cause image defects such as fog. In order to resolve such a problem, many methods have been proposed in which conductive fine particles are added as an external additive to toner particles so as to control chargeability required as the toner. For example, it is widely known to use the magnetic toner in the state that carbon black has been made to adhere or stick firmly to toner particle surfaces in order to, e.g., keep the toner from being charged in excess and make its charge distribution uniform. However, the presence of such conductive fine particles on the toner particle surfaces may on the other hand be likely to make the toner charged non-uniformly or insufficiently in environments where electric charges tend to leak as in a high-temperature and high-humidity environment. Also, the rubbing friction between toner particles themselves or between the toner and a toner layer thickness control member may cause the external additive of the toner to come off or come buried in the toner particles, resulting in low charging stability.
As having such problems, in order to make the toner have stable developing performance even in severe environments, it is studied to improve its chargeability by controlling not the external additive but raw materials for the toner and controlling the state of their dispersion.
Studies made by the present inventors have revealed that a toner inside the toner particles of which a magnetic material is locally present and on the toner particle surfaces of which any magnetic material is substantially not present has a high resistance and tends to cause the charge-up because its particle surfaces are composed of a resin. Also, where the magnetic material is locally present or stands agglomerated in toner particles, the toner may have a non-uniform chargeability. As the result, tone non-uniformity called sleeve ghost may occur on images, or low density uniformity may result on solid black images.
In order to resolve the above problems, it is also proposed to control the dielectric dissipation factor (tan δ) that is an index of the state of dispersion of a magnetic material in toner particles, to make the toner stable against any changes in developing performance with environmental variations.
In PTL 1, the particle surface properties and particle shape of a magnetic material are controlled to make the magnetic material low agglomerative so as to make the magnetic material dispersible in the whole toner particles to control the dissipation factor (tan δ) of a toner, to thereby make the toner regulated on its chargeability and improved in its developing performance. Also, in PTL 2 and PTL 3, the dielectric dissipation factor (tan δ) in a high-temperature range and that in a normal-temperature range are controlled in an attempt to make the toner less change in its chargeability with environmental variations.
However, these methods are all directed toward how the magnetic material be dispersed in the whole toner particles, and hence it has been insufficient for the magnetic material to be kept from coming bare to toner particle surfaces. If the magnetic material stands bare to toner particle surfaces, the points where it stands bare thereto serve as leak sites of electric charges to cause charge insufficiency and further make the toner have non-uniform charge quantity distribution. In such a case, selective development takes place, where only a toner having an appropriate charge quantity participates in development and a toner having a low charge quantity comes to be accumulated inside a developing assembly to cause image defects such as fog.
Meanwhile, in PTL 4 and PTL 5, a magnetic material is made present within a stated distance from toner particle surfaces and also the magnetic material is kept from coming bare to toner particle surfaces, to thereby make a toner less change in its chargeability with environmental variations. For this end, the toner is so structured that magnetic material distributed layers where the magnetic material is present at a relatively high density are present in the vicinity of particle surfaces. The presence of the magnetic material in the vicinity of particle surfaces without standing bare thereto keeps the charge-up from occurring in a low-temperature and low-humidity environment and at the same time makes the selective development less take place that may come as the charge quantity distribution becomes broad. This keeps any decrease in image density and any image defects such as fog from occurring. Further, inasmuch as the magnetic material is kept from coming bare to toner particle surfaces, the electric charges are kept from leaking in a high-temperature and high-humidity environment, to make the toner have stable chargeability against any environmental variations.
However, since the magnetic material is present at a high density in the vicinity of toner particle surfaces, magnetic material particles may agglomerate one another in the toner particles. Such agglomeration of magnetic material particles one another is considered to be caused by any mutual attraction of hydroxyl groups one another which have remained on the particle surfaces when magnetic material particle surfaces have non-uniformly hydrophobic-treated. Such a state of dispersion of the magnetic material as viewed microscopically affects the charging uniformity of the toner, so that, where the development is performed at a high speed especially in severe environments for charging as in, e.g., a high-temperature and high-humidity environment, differences in charge quantity may come between toner particles themselves to cause sleeve ghost or density non-uniformity.
In PTL 6, it is proposed to use a magnetic material in which Si element level on the magnetic material particle surfaces is specified and at the same time the magnetic material particle surfaces have been modified with a surface modifying agent, to thereby improve a toner in its environmental stability. There, however, is further room for improvement as to making magnetic material particle surfaces uniformly hydrophobic. Making the magnetic material hydrophobic affects the state of dispersion of the magnetic material in toner particles, and besides affects also the water adsorption of the toner to greatly influence the stability of developing performance in a high-temperature and high-humidity environment.